1. Field of Invention
This invention relates generally to an aircraft attitude and orientation control system. Specifically, the invention uses infrared sensors oriented about an aircraft to detect the comparative heat levels of the horizon.
The surface of the earth is typically measurably warmer than the sky due to the different heat capacities of air compared to land or water. This difference is used by the present invention to provide a reference for an aircraft regarding the horizontal attitude (pitch and roll) of the aircraft and its vertical orientation (inverted/non-inverted) to the ground. This reference is measured by comparing the electrical outputs of infrared sensors, which can be used to provide feedback to the aircraft control system to keep the aircraft in a neutral attitude (straight and level) and orientation (non-inverted). While this invention may have applications in a variety of aircraft, the preferred embodiment is described in radio controlled model airplanes and helicopters.
2. Related Art
Different sensor systems for application to controlling aircraft and spacecraft are known to the prior art. There are a variety of systems for aircraft that detect the aircraft's orientation, and then provide control feedback to keep the aircraft in its prior orientation. In full scale aircraft, these systems generally take the form of automatic flight control systems or autopilots, which utilize on-board acceleration detectors in the three orthogonal axes (lateral X, longitudinal Y, and vertical Z). They usually include built-in functions for guidance and flight direction using radio navigation, magnetic heading sensors and on-board acceleration data. However, such systems are expensive, technically complex and physically large and heavy.
There are also attitude sensor systems for satellite spacecraft. Illustrative of such methods and mechanisms is that disclosed in Doctor, U.S. Pat. No. 5,477,052 ('052 patent), which discloses a method of using focused sensors for detecting the earth's horizon from space. The '052 patent discloses an array sensor system to compensate for variations in the atmosphere when measuring IR emissions from the earth. These variations are caused by seasonal or geographic changes in the temperature or radiance of the Earth's surface. The '052 patent is directed to a method of accurately locating the interface of cold space and earth, to provide a reference point for the satellite's attitude adjustment system. It does not disclose a terrestrial based system that uses the temperature gradient from land to sky to provide an orientation system. Further, the '052 patent and those directed to satellite orientation systems detect the earth's horizon using a single field of view, and do not compare multiple fields of view to determine the satellite orientation.
A similar prior art for satellites is found in the patent issued to Diedrickson, U.S. Pat. No. 5,744,801 ('081 patent), which discloses a dual array system space horizon detection system similar to the '052 patent described above. The '081 patent expressly uses pyroelectric sensor elements, which are capacitive in nature and require the incident radiant flux to be chopped or pulsed due to the voltage decay to zero due to current flow through the internal leakage resistance. In the present invention, the infrared sensors are thermopiles, which are voltage-generating devices acting as a pure resistance, and thus do not have such capacitive limitations.
There are also prior art systems using lightweight and compact systems for use in small scale and model aircraft. One type of system is rate based, measuring the rate of change in an aircraft's attitude to compute its orientation. One rate-based system uses an inertial solid state micro-miniature guidance system to sense angular rate in the three axes. Like mechanical gyroscope systems, these systems are initially accurate, but they require continuous on-board adjustment, typically through the use of software, to compensate for the earth's rotation of 15.degree. per hour. They are expensive and have a relatively high power requirement, typically +12V at 250 mA.
Other pilot assist devices for model aircraft use position based systems that measure where the aircraft is relative to a physical reference point or area. One such position based systems uses a visible light reference. These systems operate on the assumption that the sky (up) is brighter then the earth (down). They typically use visible light sensors placed in orthogonal axes inside a translucent dome. A decrease in output levels from the sensors correlating to a decrease in relational light intensity is interpreted to be a deviation from level flight, and feedback signals are sent to the on-board control system. However, this system typically has noise from brightness on the earth. This "noise" is caused by different levels of brightness on the earth surface, which create an uneven light signature that is difficult for the system to read accurately. Furthermore, the system can obviously only be used in the daytime when the sun is clear and overhead. If used at sunrise or sunset, the system will roll the aircraft 90.degree. in an attempt to orient itself to the sun on the horizon. The system is further limited to flying conditions over dry surfaces that are relatively non-reflective of visible light.
The prior art describes either a large and expensive system for full-scale aircraft, a narrowly focused system for spacecraft to detect along a single ray the space/earth horizon, or an inefficient or expensive system for measuring a small-scale aircraft's attitude. It would be a useful improvement of the prior art for an aircraft attitude measurement system to be lightweight, compact and inexpensive that provides on-board attitude feedback information that is rapidly updated, does not need to compensate for the earth's rotation, and can be used in the day or at night. This system could provide electrical information to assist in the control of small scale and model aircraft. To achieve such improvements, this invention uses commercially available pairs of inexpensive infrared sensors in an axial heat signature summation configuration. This system can then provide information to an automatic feedback control system, or to update the calibration of a gyroscope based system.